Pretreatment system:

The reactions which occur in the water softener involve the removal of Mg⁺⁺ and Ca⁺⁺ ions both. The water softener holds resin in that the insoluble exchange site is the SO₃^- molecule, or the soluble ions attached to the exchange site are Na⁺ ions.  While water containing Mg⁺⁺, Ca⁺⁺, and HCO₃ ^- ions is passed over the resin within the softener, an ions are exchanged through the following reaction (Mg⁺⁺ removal is same).

2R-SO₃^-Na⁺ + Ca⁺⁺ + HCO₃^- → 2R-SO₃Ca⁺⁺ + 2Na⁺ + HCO₃^-                      (4-6)

(resin complex)                                 (resin complex)

Remember that electrical neutrality is managed before and after the exchange reaction. One calcium ion along with two positive charges is attached to two exchange sites which release two sodium ions with one positive charge each.  The HCO3^- ion is not affected through the reaction and passes by the resin of the softener.

To acquire pure water, it is essential to demineralize the water fully that is accomplished using a cation exchanger, an anion exchanger, and an aerator.

The cation exchanger holds resin in the hydrogen form.  Within this treatment step, essentially all cations entering the ion exchanger will be held at the exchange site, and H⁺ will be released as displays in the following classic reaction (the anions, specifically the HCO3^- ions, are unaffected through the cation exchanger).

Na⁺ +  HCO₃^- + R-  SO₃^- H    ⁺ → R- SO₃^- Na⁺ +     H⁺ +    HCO₃^-                         (4-7)

The water leaving the resin is somewhat acidic (depending on the incoming ion concentration) since it holds H+ ions and whatever anion was related along with the incoming cation.  After passing by the cation resin, HCO₃^- ions merge with the H⁺ ions to form carbonic acid (H₂CO₃).  The Carbonic acid is a weak acid which will decompose to water and CO₂ through the following reaction.

H₂CO₃ ↔ H₂O + CO₂                                                                                               (4-8)

Since the carbonic acid readily dissociated, the aerator is used to erase the CO₂ from the makeup water at this point in the system. If we aerate the water by some means, such as spraying it through a tower or blowing air through the water, the CO₂ is "stripped" from the water and vented to the atmosphere. The removal of CO₂ forces Reaction (4-8) to shift to the right, which converts more H₂CO₃ to CO₂.  With sufficient aeration, all bicarbonate (HCO₃^-), and therefore CO₂, can be removed.

Same reactions occur in the anion exchanger.  For instance, anion resin, that has hydroxide ions at the exchange sites, that will react as denoted in the following classical reaction.

H⁺ +    Cl^- + R- N (CH₃)₃⁺ OH^- →    R- N(CH₃)₃⁺ Cl^- +      HOH                          (4-9)

Within this pretreatment system, an anion resin is downstream of the cation resin, and the just cations present are hydrogen ions.  While the hydroxyl ions are released from the anion exchange sites, that merge along with the hydrogen ions to form water.  Conclude, pure water appears at the effluent (that is somewhat overstated since an extremely small amount of other cations and anions pass unaffected by the resin columns in original practice).